1. Field of the Invention
The present invention relates to a recording control apparatus, a recording and reproduction apparatus, and a recording control method using a maximum likelihood decoding method.
2. Description of the Related Art
In recording and reproduction apparatuses for recording original digital information on, or reproducing such information from, a portable recording medium, there can be a variance in the shape of marks formed on the medium among individual apparatuses or recording mediums even with an identical shape of recording pulse. This results in significant difference in the quality of the signal reproduced. In order to avoid reduction in the reliability due to the variance, a correction operation is performed when, for example, the recording medium is mounted. A correction operation is a control operation for optimizing the setting of characteristics of the reproduction system, the shape of the recording pulse, or the like, in order to guarantee the reliability of user data.
A general information reproduction apparatus includes a PLL circuit for extracting clock information included in a reproduction signal and identifying the original digital information based on the clock information extracted.
FIG. 14 shows a conventional optical disc drive. Light reflected by an optical disc 17 is converted into a reproduction signal by an optical head 18. The reproduction signal is shape-rectified by a waveform equalizer 19. The resultant reproduction signal is binarized by a comparator 20. Usually, the threshold of the comparator 20 is feedback-controlled such that an accumulation result of binary signal outputs is 0. A phase comparator 21 obtains phase errors between the binary signal outputs and the reproduction clocks. The phase errors are averaged by an LPF 22, and a control voltage of a VCO 23 is determined based on the processing result. The phase comparator 21 is feedback-controlled such that the phase errors output by the phase comparator 21 are always 0. In recording mediums on which information is thermally recorded, the shape of the marks formed thereon vary in accordance with the thermal interference of the mediums and recording patterns before and after the mark which is to be recorded. Therefore, a recording parameter which is optimal for the recording of each pattern needs to be set.
The above-described error detection output is an index for evaluating the recording parameter. The recording parameter is set such that the error detection output is as small as possible. Specifically, a recording compensation circuit 27 generates a pulse having a prescribed pattern based on a recording pattern which is output from a pattern generation circuit 26 using an initially set recording parameter. A laser driving circuit 28 records information on the optical disc. While information is being reproduced from a track having the prescribed pattern recorded thereon, an error detection circuit 24 accumulates absolute values of phase errors between an output from the comparator 20 and an output from the VCO 23, and thus obtains a detection signal. The detection signal is correlated with jitter between a reproduction clock and a binarized pulse edge. Recording and reproduction are repeatedly performed with different recording parameters. The recording parameter used when the detection value is minimal is determined as an optimal recording parameter.
FIG. 15 shows a specific operation of the error detection circuit 24. Here, a recording pattern having a repetition of 6 T, 4 T, 6 T and 8 T is used. The mark termination edge corresponding to a pattern of a combination of 4 T marks and 6 T spaces is optimized. It is assumed that a mark start edge corresponding to a pattern of a combination of 6 T spaces and 8 T marks, and a mark termination edge corresponding to a pattern of a combination of 8 T marks and 6 T spaces, are recorded with an optimal recording parameter.
When given an NRZI signal having a period shown in part (a) of FIG. 15, the recording compensation circuit 27 generates a laser waveform pulse shown in part (b) of FIG. 15. Tsfp is a parameter for setting a mark start position, and Telp is a parameter for setting a mark termination position. The laser driving circuit 28 modulates light emitting power in accordance with the pattern shown in part (b) of FIG. 15. An amorphous area is physically formed on the track as shown in part (c) of FIG. 15 by laser light. When Telp is varied as Telp1, Telp2 and Telp3, the shape of the mark formed is changed as shown in part (c) of FIG. 15. Information reproduction from the track having such marks will be discussed.
When the recording parameter at the end of the 4 T mark is Telp2, which is the optimal value, a reproduction signal shown with a solid line in part (d) of FIG. 15 is obtained. The threshold value is defined such that the accumulation value of the outputs from the comparator 20 is 0. A phase difference between the output from the comparator 20 and the reproduction clock is detected, and a reproduction clock (part (e) of FIG. 15) is generated such that the accumulation value of the phase errors is 0.
In the case where the recording parameter at the end of a 4 T mark is made Telp1, which is smaller than the optimal value, a reproduction signal shown in part (f) of FIG. 15 with the solid line is obtained. Since the termination edge of the 4 T mark changes in a time axis direction, the threshold value Tv of the comparator 20 is greater than in the reproduction signal shown in part (d) of FIG. 15, as indicated by the one-dot chain line in part (f) of FIG. 15. Because of the change in the output from the comparator 20, the phase of the reproduction clock is advanced as compared to a reproduction clock shown in part (e) of FIG. 15 such that the accumulation value of the phase errors is 0. As a result, a reproduction clock shown in part (g) of FIG. 15 is generated.
In the case where the recording parameter at the end of a 4 T mark is made Telp3, which is greater than the optimal value, a reproduction signal shown in part (h) of FIG. 15 with the solid line is obtained. Since the termination edge of the 4 T mark changes in a time axis direction, the threshold value Tv of the comparator 20 is smaller than in the reproduction signal shown in part (d) of FIG. 15, as indicated by the one-dot chain line in part (h) of FIG. 15. Because of the change in the output from the comparator 20, the phase of the reproduction clock is behind as compared to a reproduction clock shown in part (e) of FIG. 15 such that the accumulation value of the phase errors is 0. As a result, a reproduction clock shown in part (i) of FIG. 15 is generated.
Measurement results of the time difference between the mark termination edge (rising edge of a reproduction signal) and the reproduction clock (so-called data-clock jitter) exhibit distributions shown in parts (j) through (l) of FIG. 15. It is assumed here that the 4 T mark termination edge and the 8 T mark termination edge have a variance such that both of the edges exhibit normal distributions of identical variance values.
In the case of the reproduction signal shown in part (d) of FIG. 15, and the reproduction clock shown in part (e) of FIG. 15, the time difference distribution between the output from the comparator 20 and the reproduction clock at the mark termination edge (rising edge) is as shown in part (k) of FIG. 15. The average value of the distributed values at the 4 T mark termination edge, and the average value of the distributed values at the 8 T mark termination edge, are each 0.
In the case where the recording parameter of the end of the 4 T mark is Telp1 (smaller than the optimal value Telp2), neither the average value of the distributed values at the 4 T mark termination edge, nor the average value of the distributed values at the 8 T mark termination edge, is 0, but both are away from 0 by the same distance, as shown in part (j) of FIG. 15. Therefore, the total variance at the rising edge is greater than the case in part (k) of FIG. 15.
In the case where the recording parameter of the end of the 4 T mark is Telp3 (greater than the optimal value Telp2), neither the average value of the distributed values at the 4 T mark termination edge, nor the average value of the distributed values at the 8 T mark termination edge, is 0, but both are away from 0 by the same distance, as shown in part (l) of FIG. 15. In part (j) and part (l) of FIG. 15, the distribution of the 4 T mark termination edge and the distribution of the 8 T mark termination edge are inverted. In this case also, the total variance at the rising edge is greater than the case in part (k) of FIG. 15.
In the case where the accumulation result of absolute values of phase errors is the error detection output, the error detection value changes as shown in part (m) of FIG. 15 in accordance with the change in the recording parameter Telp. Accordingly, the recording parameter is varied, and the recording parameter when the output from the error detection circuit 24 is minimal is determined as an optimal recording parameter.
In the above example, the recording parameter Telp at the 4 T mark termination edge is optimized. For the other recording parameters, test recordings using a respective specific parameter are performed and the optimal recording parameters are obtained based on the error detection output.
FIG. 16 is a flowchart illustrating an operation for obtaining all the recording parameters in accordance with the above-described procedure. Areas of a medium on which test recordings are to be performed are accessed (S161), and the test recordings are performed while the recording parameter at the mark start edge or the mark termination edge is changed prescribed area by prescribed area (for example, sector by sector) (S163). Information is reproduced from the test recording areas, and error detection outputs are obtained area by area by which the recording parameter is changed (S164). The recording parameter at which the error detection output is minimal is determined as an optimal parameter (S165). This operation is repeated until all the optimal parameters are obtained (S162) (see Japanese Laid-Open Publications Nos. 2000-200418 and 2001-109597).
The above-described method by which the recording parameter is set such that the jitter is minimal has the following problem. In a system adopting the maximum likelihood decoding method, the probability of error generation is not necessarily minimal. Typically by the maximum likelihood decoding method, a signal pattern is estimated from a reproduction signal waveform, and a reproduction signal waveform and the estimated signal waveform are compared with each other, so that the reproduction signal is decoded into a signal having a signal pattern which has the maximum likelihood. By the maximum likelihood decoding method, the probability of error generation is lower as the difference between the reproduction signal waveform and the estimated signal waveform is smaller.